Photovoltaic devices regularly exhibit performance degradation due to the presence of current shunts. These shunts may be induced at various points in the process, such as problems with initial crystalline quality, contact over-firing, surface conduction at crack boundaries, trace metal deposition at the wafer edge, etc. The resultant defect population usually causes a relatively minor overall degradation of performance of a device that is properly characterized and accounted for during the final test and sort. It is important to continuously reduce the shunt defect population to improve end-of-line yield and move more cells to higher performance bins, but a more significant effect must also be considered that involves long term field reliability.
Inline shunt detection becomes particularly important when one considers the effect of reverse-bias induced shading on modules installed in the field. In the extreme case a single cell is shaded by a branch, bird or other object for some period of time and the cell is placed into strong reverse bias by its series-connected neighbors. It is not unusual for a conventional 150 mm silicon cell to be reverse-biased to more than 10V, and flow 5 A of current during the time that it is shaded. Many cells are able to survive the reverse bias condition without damage, but others experience some degree of damage as described below.
A conventional silicon cell which exhibits reasonable reverse bias current density uniformity will usually exhibit no measurable damage during the stress period. In this case, the heat load is well distributed across the cell and there is only a mild rise in local cell surface temperature. Cells that contain extensive networks of relatively small shunts may avoid degradation during reverse bias operation, where a large but well-distributed defect density effectively uses the surrounding silicon as a temperature sink to prevent local overheating. This type of cell may exhibit relatively poor efficiency at end of line test due to the network of shunts, but it will not suffer from further degradation due to the shade-induced field degradation mechanism.
Conversely, a cell with similar performance and J-V characteristics as the cell described above may be damaged if the shunts are concentrated in a localized area, or if the shunt defects are large. In this case, the entire 5 A of reverse bias current flows through a smaller area of silicon, and the higher current density (expressed in Amps/cm2) may cause a local temperature rise that damages the cell within seconds. Contact de-lamination, discoloration due to transparent conductive oxide (TCO) damage, and melting of encapsulation layers are all readily observed on a variety of commercial cells, both conventional silicon and thin film, due to this effect. The localized heating may also compromise the module lamination layer, allowing moisture to enter the cell and cause freeze/thaw damage or accelerated corrosion. This damage often results in permanent degradation of cell properties: the cell no longer performs per the original J-V test results, and because the cell is series-connected in the module, the module efficiency is proportionally reduced. A similar effect occurs in thin-film cells which are monolithically grown and scribed into series-connected cells: partial shading may reverse bias one cell and cause overheating in regions where severe shunts exist. Thin-film cells often have even poorer thermal conduction characteristics than traditional bulk photovoltaic materials, and so may be more susceptible to this type of field degradation mechanism than the first generation of cells.
Cell degradation due to localized heating described above has been known for some time, but is becoming increasingly important due to the trend toward utility-scale PV contracts and purchase agreements. In years past, the predominant PV customers (homeowners) were unlikely to detect a moderate decrease of module performance year over year. Modern utility-scale installations, however, depend on Power Purchase Agreements (PPA's) that specify the precise power output of each module. New technologies are being developed to monitor the power output of every module (and in some cases even sub-module) so that the PPA's can be effectively enforced.
When PPA's are in effect, cell manufacturers benefit financially if aggressive performance guarantees can be signed, and are penalized when modules fail to meet the long-term performance goals in the field. Both parties benefit when field degradation mechanisms are controlled and eliminated.
Due to the Severity of the shading-induced mechanism discussed here, what is needed is an inline-screening test to detect cells exhibiting spatially non-uniform current flow. The solution must have high throughput (15 cells per minute or faster), must characterize the entire cell (full wafer imaging) and should be cost effective if it is to be used as an inline inspection step in the manufacturing line.